In some countries, acetylcholinesterase inhibitors such as physostigmine or tetrahydroaminoacridine (THA) are used as preventive or therapeutic drugs for Alzheimer's disease. However, successful results have not been obtained. It has also been attempted to develop 5-HT agonists acting on the nerve e system, but their mechanism has not yet been completely elucidated.
Pathological traits of Alzheimer's disease include senile plaques and accumulation of PHFs (paired helical filaments) in the brain.
The .beta.-amyloid protein (hereinafter referred to as A.beta.), which is the main component of senile plaques, is an insoluble peptide constituted by approximately 40 amino acid residues. It has been elucidated that A.beta. itself damages neurons biochemically, and particularly that coagulated A.beta.--not solubilized A.beta.--is responsible for this effect.
Thus, it has been desired that a substance useful for preventing the progress of or for curing Alzheimer type senile dementia be discovered and developed as a pharmaceutical based on the mechanism of suppressing death of neurons incurred by toxic A.beta..